monte carlo 2003 ls low idle

3.4 v6 ls 40,000 kms 2003 when i stop the idle on the tac goes below 500 rpms then shoots up then goes back down and ussually hovers around 600 - 650 rpms
i had it in the shop its still under warrantee they cant find the problem....the ses light doesnt come on. what could be the problem?
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